Centrally located access alarm system

ABSTRACT

Apparatus for monitoring a plurality of remotely located doors or conditions in a building. A separate sensor is positioned adjacent each door or at each location at which a condition is sensed. Each sensor or group of connected sensors has a resistor which is connected to a centrally located detector circuit by means of a separate pair of electrical conductors. Each sensor includes means for disconnecting the resistor from the connected pair of conductors either when the adjacent door is open, when some other monitored condition is sensed, or when the sensor is tampered with. The detector circuit includes two relaxation oscillators. The resistor in one or more sensors forms a portion of an RC timing circuit in a first of the oscillators. A change in the resistance in the sensor changes the operating frequency of the first oscillator to cause the second oscillator to cycle and trigger an alarm.

mte States Patent [151 3,641,552

Friberg Feb. 8, 1972 [54] CENTRALLY LOCATED ACCESS 2,928,081 3/ 1960 Grosso.... ..340/276 ALARM SYSTEM 2,085,773 7/1937 Squire.. ..340 409 x 2,912,540 1 1/ 1959 'Sawicki ..340/274 X [72] Inventor: Nelson A. Frlberg, Toledo, Ohio 7 73 Ass ee: International Asemblix c a WWW-David Tram" 1 lg Toledo i 0mm on Attorney-Owen 81'. Owen 22 Filed: Feb. 18, 1970 [s71 ABSTRACT [21] Appl. No.: 12,401 Apparatus for monitoring a plurality of remotely located doors or conditions in a building; A separate sensor is positioned adjacent each door or at each location at which a con- [52] U.S.Cl. dmon is sensed. Each sensor or g p of connected Sensors [51] km Cl 6 13/22 has a resistor which is connected to a centrally located detec- [58] Field 331/64 tor circuit by means of a separate pair of electrical conduc- 331/65 tors. Each sensor includes means for disconnecting the resistor from the connected pair of conductors either when the adjacent door is open, when some other monitored condition [56] cited is sensed, or when the sensor is tampered with. The detector UNITED STATES NT circuit includes two relaxation oscillators. The resistor in one or more sensors forms a portion of an RC timing circuit in a 2,877,361 3/ 1959 Chase ..340/274 X fi t f the 0Sci||ators A change in the resistance in the sensor Fmh. I u 340/ changes the operating frequency of the oscillator to cause 3,553,730 1/1971 Kaplm et aim. .....340/274 X the second oscillator to cycle and trigger an alarm 3,171,112 2/1965' .....340/276 X 3,010,100 11/1961 Muehter ..340/276 9 Claims, 6 Drawing Figures TEST 3; 34 //3 bur Q i /6 l '33 l a 4/ 6/I35 J l 7" a; 7 60 aa 20 2 PAIENTEUFEB a ma 3.2341552 SHEET 1 OF 2 INVENTOR: NELSUNA. 1321155125.

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CENTRALLY LOCATED ACCESS ALARM SYSTEM BACKGROUND OF THE INVENTION This invention relates to alarm systems and more particu larly to a centrally located alarm system for simultaneously monitoring a plurality of remotely located doors or conditions in a building.

Many businesses, such as retail department stores, have had a recurring problem with employees carrying stolen merchandise through unsupervised back doors. One solution has been to prove a guard at each door. However, this is prohibitively expensive, particularly where there are a number of widely separated doors which are rarely used, such an emergency fire exits. In these cases, a mechanical alarm is often placed on each door. The mechanical alarm may have a spring-operated bell which is actuated when the door is opened. Authorized personnel have special keys for disabling the alarm prior to opening the door. The major drawbacks to this arrangement are that the special keys may be duplicated and, where the door is at a remote location, the alarm may not be heard or reported until it is too late to prevent a theft.

' In' an improved alarm system, a sensor is positioned adjacent each door and is connected to a centrally located alarm. Typically, the sensor is a simple switch which either makes or breaks a circuit when the door is opened. This system may, however, be easily defeated by either opening or shorting the circuit to'bypass the sensor. In many installations, the sensor comprises a reed switch which is closed when the door is closed by a magnet mounted on the door. If a bar magnet is held adjacent the sensor, the reed switch can be held closed, even though the door is opened, thus defeating the alarm system.

SUMMARY OF THE INVENTION According to the instant invention, an improved alarm system is provided for simultaneously monitoring a plurality of remotely located doors or other stations or conditions in a buildingThe alarm system is provided with a plurality of sensors, one of which is mounted adjacent each door or station being monitored. Each sensor includes a resistor which is connectedto a centrally located detector circuit by a separate pair of conductors. If a door is opened or if the sensor is tampered with, switches in the sensor disconnect the resistor from the connected pair of conductors either by shorting out the resistor or by opening the circuit between the resistor and the conductors. Should the detector circuit fail to detect the presence of a sensor resistor across any pair of conductors, a first indicator lamp associated with such pair of conductors and an audible alarm are energized. A switch permits an operator to substitute a second indicator lamp for the audible alarm and the first indicator lamp when the door must be opened for an authorized purpose. The second indicator lamp, which is preferably a different color from the first indicator lamp, will remain on only while the door is open. Thus, the detector circuit is responsive to opening a monitored door, to tampering with a sensor, to shorting together the pair of conductors, and to cutting the pair of conductors, all of which effectively disconnect the sensor resistor from the associated detector circuit.

A separate detector circuit is used for each sensor or each channel which may include several sensors. The detector circuit generally comprises first and second astable unijunction oscillators of the relaxation type. The first relaxation oscillator in each channel operates at a frequency which is determined by the remotely located sensor resistor, while the second relaxation oscillator operates at a lower fixed frequency. As long as the sensor resistor appears across an associated pair of conductors, the first oscillator will operate at its normal frequency and keep the second oscillator from operating. If, however, the resistor is disconnected from the conductors or the effective resistance is changed, the first oscillator will stop or slow down to a frequency lower than the fixed frequency of the second oscillator, permitting the second oscillator to operate. Operation of the second oscillator fires a silicon-controlled rectifier which energizes the alarm.

The alarm, or at least one channel of a multichannel alarm, may also be used for a fire alarm. A number of sensors having heat-actuated switches or fuses may be connected in series or parallel with a single resistor. The resistor may be connected in one of the sensors or any suitable location across the pair of conductors which connect the sensors to the detector circuit. When heat actuates a sensor, the resistor is disconnected from the detector circuit either by shorting or opening the conductors. The fire alarm will be effective even if fire damages the pair of conductors, since the alarm is triggered by any shorting or opening of the conductors.

Accordingly, it is a primary object of the invention to provide an improved centrally located alarm system for monitoring a plurality of remotely located doors or conditions.

Another object of the invention is to provide an improved alarm system responsive to a remotely located sensor, to the shorting together of a pair of conductors connected to such sensor and the cutting of the conductors connected to such sensor.

Other objects and advantages of the invention will become apparent from the following detailed description, reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3 is an end elevational view in section of the sensor of 4 FIG. 2;

FIG. 4 is an end elevational view in section, similar to FIG. 3, and showing a modified embodiment of the sensor;

FIG. 5 is a schematic circuit diagram of an alarm system according to the instant invention; and

FIG. 6 is a graph showing operating voltages at various points in the circuit of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1-3, a novel sensor 11 is shown for monitoring a remotely located door 12. The sensor 11 includes a normally open reed switch 13 which is held closed by a magnet 14 when the door 12 is closed. The switch 13 is connected in series with a resistor 15 across the ends of a pair of conductors 16 and 17. The sensor 11 has a nonmagnetic enclosed housing 18 to permit actuation of the switch 13 by the door magnet 14. A magnetic shield 19 is positioned in the housing 18 to shield the reed switch 13 from external magnetic fields from magnets other than'the door magnet 14. A second normally open reed switch 20 is connected between the conductors l6 and 17, in parallel with the series switch 13 and resistor 15. If an external magnet is brought near the sensor 11 in an attempt to disable the sensor 11 for an unauthorized use of the. door .12, the switch 20 is closed to short out or disconnect the resistor 15 from the conductors 16 and 17. The embodiment of the sensor 11 shown in FIGS. 2 and 3 has a removable cover 21 for enclosing the housing 18. Mounting screws 22 for attaching the sensor 11 to a door frame 23 are located within the enclosed housing 18. When the cover 21 is removed from the sensor 11 to permit access to the mounting screws 22, the switches 13 and 20 and the resistor 15, a push button switch 24 is closed to short out or disconnect the resistor 15 from the conductors l6 and 17. Thus, when the door 12 is in its normally closed position, the switch 13 is closed to connect the resistor 15 across the ends of the conductors l6 and 17. The resistor 15 will continue to appear across the conductors l6 and 17 unless the door 12 is opened, an auxiliary magnet is brought adjacent the sensor 1 l or the sensor 11 is opened. It will be appreciated that the sensor 11 may be modified for use as fire detector by replacing either the normally closed switch 13 with a fuse or other switch which opens in response to heat or the normally open switch with a switch which closes in response to heat.

A modified embodiment of the sensor 11 is shown in FIG. 4. The modified sensor 11 is identical to the embodiment of FIGS. 2 and 3, except that the sensor 11 has a housing which has a removable bottom access plate 26. Exposed screws 27 are used to attach the sensor 11 to the door frame 23. If the screws 27 are removed, a plunger 28 is released to close the switch 24, thereby disconnecting the resistor 15 from the conductors l6 and 17.

Referring now to FIG. 5, a detailed schematic circuit diagram is shown for the alarm system. The schematic is shown having two separate channels, each connected to a separate sensor 11. However, it is to be understood that any number of similar channels may be connected in parallel for monitoring any number of doors, windows or conditions. A suitable power supply, such as a power transformer 30 and a full-wave rectifier 31, apply a pulsating direct current between a common chassis ground 32 and a buss 33. The buss 33 is connected to apply a positive voltage through a diode 34 to a buss 35. The voltage appearing on the buss 35 is filtered by a filter capacitor 36 to provide a direct current having a minimum amount of ripple. The buss 35 is connected through a normally closed test switch 37 to the conductors 16 extending to each of the sensors 11. The conductor 17 from each sensor 11 is connected to a resistor 38 in an associated channel. The resistor 38 is in turn connected to a common junction between the emitter of an unijunction transistor 39 and a capacitor 40. The

capacitor 40 is also connected to the common ground 32,

while the unijunction transistor 39 has a base connected through a resistor 41 to the positive buss 35 and a second base connected through a resistor 42 to the common ground 32, thus forming a relaxation oscillator circuit. As long as the door 12 is kept closed, the reed switch 13 will be held closed to allow a charging current to flow from the buss 35 through the conductor 16, the resistor 15, the conductor 17, and the resistor 38 to the capacitor 40. When the voltage across the capacitor 40 reaches a critical level, the unijunction transistor 39 will conduct to discharge the capacitor 40. The cycling rate of the unijunction transistor 39 is determined by the R-C time constant of the series connected resistor 15, resistor 38 and capacitor 40. The purpose of the resistor 38 is to protect the transistor 39 from excessive emitter currents in the event that the resistor 15 is shorted out by closing either switch 20 or switch 24 or by shorting together the conductors 16 and 17.

A second unijunction transistor 45 is connected to form a second relaxation oscillator. The emitter of the unijunction transistor 45 is connected through a resistor 46 to the positive buss 35 and through a capacitor 47 to the common ground 32. One base of the unijunction transistor 45 is connected through a resistor 48 to the positive buss 35 and the second base is connected through a resistor 49 to the common ground 32. The operating frequency of the unijunction transistor 45, as determined by the RC time constant of the resistor 46 and the capacitor 47, is established at a lower frequency than the operating frequency of the unijunction transistor 39, for example, at one-half the normal operating frequency of the transistor 39. The output of the unijunction transistor 39, which appears across the resistor 42, is applied through a DC blocking capacitor 50 to the base of an NPN-transistor 51. The transistor 51 has a collector connected to the common junction between the resistor 46, the emitter of the transistor 45 and the capacitor 47, and an emitter connected to the common ground terminal 32. Thus, each time the first relaxation oscillator cycles and the unijunction transistor 39 is momentarily turned on, a positive pulse is applied through the capacitor 50 to the base of the transistor 51 to momentarily bias the transistor 51 into a conducting state. The transistor 51 will discharge the capacitor 47, thereby preventing the second oscillator from cycling. The operation of the alarm may be more readily understood by referring to the wave forms shown in FIG. 6. The voltage across the capacitor 40 has a generally saw-toothed wave form. The capacitor 40 is charged through the series resistors 15 and 38 until a critical emitter voltage 54 for the unijunction transistor 39 is reached. When the capacitor 40 is charged to the critical emitter voltage 54 for the unijunction transistor 39 is reached. When the capacitor 40 is charged to the critical voltage 54, the unijunction transistor 39 conducts until the capacitor 40 is substantially discharged, and thereafter the cycle is repeated. The capacitor 47 will similarly charge through the resistor 46, but at a much slower rate. However, conduction of the unijunction transistor 39 and of the NPN-transistor 51 discharge the capacitor 47 before the voltage on the capacitor 47 reaches a critical level 55 for triggering the unijunction transistor 45. If, however, the resistor 15 in the sensor 11 is disconnected from the conductors l6 and 17, either by shorting or by opening the circuit, the unijunction transistor 39 will cease operating and the capacitor 47 will charge to the critical emitter voltage 55 for triggering the unijunction transistor 45. The unijunction transistor 45 conducts while the capacitor 47 is discharged, and simultaneously applies a positive triggering signal to the gate of a silicon-controlled rectifier 56.

The controlled rectifier 56 is normally connected through a switch 57 to an indicatorlamp 58 and through an isolation diode 59 to a common alarm buss 60 for energizing an audible alarm 61. The indicator lamp 58 is preferably of an attention attracting color such as red, while the audible alarm 61 may he a conventional bell, buzzer or audio oscillator circuit. Once the controlled rectifier 56 is triggered, the lamp 58 and the alarm 61 will continue to be energized, even though the door 12 may have been closed.

If the door 12 is to be used by authorized personnel, the switch 57 is placed in standby" position to connect the silicon-controlled rectifier 56 to a second indicator lamp 62 and to disconnect the lamp 58 and the audible alarm 61. The lamp 62 is connected to the pulsating direct current buss 33. Thus, when the door 12 is again closed and the unijunction transistor 39 is cycling at its normal frequency, the rectifier 56 will be shut off when the pulsating voltage on the buss 33 goes to zero.

The test switch 37 is placed in series between the positive buss 35 and all conductors 16 leading to the sensors 11. When the test switch 37 is open, each channel will be triggered for simultaneously testing the circuitry, the indicator lamps 58 and 62 and the audible alarm '61.

Although the alann system has been described for use in monitoring a plurality of doors in a building, it may he used for monitoring other conditions. For example, the sensor 11 may be connected for monitoring a plurality of windows or a combination of windows and doors or for monitoring temperature by substituting either a thermistor for the resistor 15 or a bimetallic switch for the reed switch 13. It will be appreciated that various other modifications and changes may be made in the remainder of the elements without departing from the scope of the appended claims.

What I claim is:

1. Apparatus for indicating the occurrence of a predetermined condition at a remote location comprising, in combination, a sensor mounted at the remote location, a resistor mounted in said sensor, a pair of conductors having first and second ends, means electrically connecting said resistor across said first ends of said conductors, means for disconnecting said resistor from across said conductors during the occurrence of the predetermined condition, an alarm, and a detector circuit connecting said second ends of said conductors to said alarm, said detector circuit comprising a first relaxation oscillator, said first oscillator including a capacitor and said resistor for establishing a first operating frequency, said first oscillator cycling each time said capacitor is charged through said resistor to a predetermined voltage, a second relaxation oscillator including a second resistor and a second capacitor for establishing a second operating frequency lower than said first operating frequency, said second oscillator cycling each time said second capacitor is charged to a predetermined voltage, means for discharging said second capacitor each time said first oscillator cycles, and means for energizing said alarm when said second oscillator cycles.

2. Apparatus for indicating the occurrence of a predetermined condition at a remote location, as defined in claim 1, wherein said means for energizing said alarm includes a silicon controlled rectifier connected in-series with said alarm, and means for applying a triggering signal to said controlled rectifier each time said second multivibrator oscillator cycles.

3. Apparatus for indicating the occurrence of a predetermined condition at a remote location, as defined in claim 1, whereinsaid means for disconnecting said resistor from said conductors during the occurrence of the predetermined condition includes a switch electrically connected in parallel with said resistor, and means for closing said switch during the occurrence of the predetermined condition to short said resistor.

4. Apparatus for indicating the occurrence of a predetermined condition at a remote location, as defined in claim 1, wherein said means for disconnecting said resistor from said conductors during the occurrence of the predetermined condition includes a switch electrically connected in series with said resistor and said first ends of said conductors, and means for opening said switch during the occurrence of the predetermined condition.

5. A centrally located alarm'system for monitoring a plurality of accesses to an area comprising, in combination, a plurality of sensors, each of said sensors including a resistor, means for mounting each of said sensors adjacent a different one of the accesses, a plurality of pairs of conductors having first and second ends, means connecting the first ends of each of said pairs of conductors to the resistor in a different one of 'said sensors, means in each sensor for disconnecting the resistor in such sensor from across the connected pair of conductors when the adjacent access is opened, an alarm, and a plurality of detector circuits, a different one for connecting the second ends of each of said pair of conductors to said alarm, each detector circuit comprising a first relaxation oscillator, said oscillator including a capacitor and the resistor in the connected one of said sensors for establishing a first operating frequency, said first oscillator cycling each time said capacitor is charged through such resistor to a predetermined voltage, a second relaxation oscillator including a second resistor and a second capacitor for establishing a second operating frequency lower than said first operating frequency, said second oscillator cycling each time said second capacitor is charged to a predetermined voltage, means for discharging said second capacitor each time said first oscillator cycles, and means for energizing said alarm when said second oscillator cycles.

6. A centrally located alarm system for monitoring a plurality of accesses to an area, as defined in claim 5, wherein said means in each sensor for disconnecting the resistor in such sensor from the connected pair of conductors comprising a magnetically actuated reed switch connected to disconnect such resistor when the adjacent access is opened, and magnet means for actuating said reed switch whenever the adjacent access is closed, and wherein each of said sensors further includes means responsive to a magnet other than said magnet means for disconnecting said resistor from the connected pair of conductors.

7 A centrally located alarm system for monitoring a plurality of accesses to an area, as defined in claim 6, wherein each of said sensors has a removable cover, and means in each sensor responsive to the-removal of the cover for such sensor for disconnecting the resistor in such sensor from the connected pair of conductors.

8. A centrally located alarm system for monitoring a plurality of accesses to an area, as defined in claim 5, and including means for simultaneously disconnecting at least one conductor in each of said pairs of conductors from the connected detector circuit forsimultaneously testing said detector circuits and said alarm.

9. A centrally located alarm system for monitoring a plurality of accesses of an area, as defined in claim 5, wherein said alarm includes an audible alarm and a plurality of indicator lamps, each of said lamps being associated with a different one of said detector circuits,'and wherein said means in each of said detector circuits for energizing said alarm when said second oscillator cycles energizes said audible alarm and the lamp associated with such detector circuit.

I some 1 UNITED STATES PATENT OFFICE (569 CERTIFICATE OF CORRECTION Patent No. ,552 7 Dated February 8, 1972 Inventor(s) Nelson A. Friberq It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line' 11, the word "prove" should be corrected to U read provide Column 2, line 20, after the word "and", insert the word to line 74, after the word "as", insert the word Column 4, line 3, after the word "the", first occurrence,

' delete "critical emitter voltage 54' for the";

line 4, delete" "unijunction transistor 39 is reached. When the capacitor 40 is"; line 5, delete "charged to the"; line 32, after the word "in", insert the word a Column 5, line 8 (claim 2), after the word "second", delete the word "multivibrator".

Column 6, -line 33, (claim 9), after the word "accesses", delete the word "of" and insert the word to Signed and sealed this 27th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. v ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. Apparatus for indicating the occurrence of a predetermined condition at a remote location comprising, in combination, a sensor mounted at the remote location, a resistor mounted in said sensor, a pair of conductors having first and second ends, means electrically connecting said resistor across said first ends of said conductors, means for disconnecting said resistor from across said conductors during the occurrence of the predetermined condition, an alarm, and a detector circuit connecting said second ends of said conductors to said alarm, said detector circuit comprising a first relaxation oscillator, said first oscillator including a capacitor and said resistor for establishing a first operating frequency, said first oscillator cycling each time said capacitor is charged through said resistor to a predetermined voltage, a second relaxation oscillator including a second resistor and a second capacitor for establishing a second operating frequency lower than said first operating frequency, said second oscillator cycling each time said second capacitor is charged to a predetermined voltage, means for discharging said second capacitor each time said first oscillator cycles, and means for energizing said alarm when said second oscillator cycles.
 2. Apparatus for indicating the occurrence of a predetermined condition at a remote location, as defined in claim 1, wherein said means for energizing said alarm includes a silicon controlled rectifier connected in series with said alarm, and means for applying a triggering signal to said controlled rectifier each time said second multivibrator oscillator cycles.
 3. Apparatus for indicating the occurrence of a predetermined condition at a remote location, as defined in claim 1, wherein said means for disconnecting said resistor from said conductors during the occurrence of the predetermined condition includes a switch electrically connected in parallel with said resistor, and means for closing said switch during the occurrence of the predetermined condition to short said resistor.
 4. Apparatus for indicating the occurrence of a predetermined condition at a remote location, as defined in claim 1, wherein said means for disconnecting said resistor from said conductors during the occurrence of the predetermined condition includes a switch electrically connected in series with said resistor and said first ends of said conductors, and means for opening said switch during the occurrence of the predetermined condition.
 5. A centrally located alarm system for monitoring a plurality of accesses to an area comprising, in combination, a plurality of sensors, each of said sensors including a resistor, means for mounting each of said sensors adjacent a different one of the accesses, a plurality of pairs of conductors having first and second ends, means connecting the first ends of each of said pairs of conductors to the resistor in a different One of said sensors, means in each sensor for disconnecting the resistor in such sensor from across the connected pair of conductors when the adjacent access is opened, an alarm, and a plurality of detector circuits, a different one for connecting the second ends of each of said pair of conductors to said alarm, each detector circuit comprising a first relaxation oscillator, said oscillator including a capacitor and the resistor in the connected one of said sensors for establishing a first operating frequency, said first oscillator cycling each time said capacitor is charged through such resistor to a predetermined voltage, a second relaxation oscillator including a second resistor and a second capacitor for establishing a second operating frequency lower than said first operating frequency, said second oscillator cycling each time said second capacitor is charged to a predetermined voltage, means for discharging said second capacitor each time said first oscillator cycles, and means for energizing said alarm when said second oscillator cycles.
 6. A centrally located alarm system for monitoring a plurality of accesses to an area, as defined in claim 5, wherein said means in each sensor for disconnecting the resistor in such sensor from the connected pair of conductors comprising a magnetically actuated reed switch connected to disconnect such resistor when the adjacent access is opened, and magnet means for actuating said reed switch whenever the adjacent access is closed, and wherein each of said sensors further includes means responsive to a magnet other than said magnet means for disconnecting said resistor from the connected pair of conductors.
 7. A centrally located alarm system for monitoring a plurality of accesses to an area, as defined in claim 6, wherein each of said sensors has a removable cover, and means in each sensor responsive to the removal of the cover for such sensor for disconnecting the resistor in such sensor from the connected pair of conductors.
 8. A centrally located alarm system for monitoring a plurality of accesses to an area, as defined in claim 5, and including means for simultaneously disconnecting at least one conductor in each of said pairs of conductors from the connected detector circuit for simultaneously testing said detector circuits and said alarm.
 9. A centrally located alarm system for monitoring a plurality of accesses of an area, as defined in claim 5, wherein said alarm includes an audible alarm and a plurality of indicator lamps, each of said lamps being associated with a different one of said detector circuits, and wherein said means in each of said detector circuits for energizing said alarm when said second oscillator cycles energizes said audible alarm and the lamp associated with such detector circuit. 